A retool - not the same as a reboot - is when a show stays within the same continuity, but decides to change things fairly drastically in a way that is more influenced by events behind the camera, rather than any logical consequence of the show's story.

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Low ratings, unpopular characters, or changing to better adapt to competition are common reasons for a retool to happen. Retools often involve shifts in the show's storyline, changing the show's premise or characters, or even changing the show's entire format. Often, these happen between seasons, as a natural stopping point. In extreme circumstances, sometimes it simply happens mid-season, with one episode suddenly being very different from what came before.

10 The Mentalist Shook Things Up In Season 6

Patrick Jane and Teresa Lisbon following the timeskip The Mentalist

For its first five seasons, The Mentalist had several ongoing storylines - including the character development of Patrick Jane, his relationship with Teresa Lisbon, and the hunt for serial killer Red John. The Mentalist's sixth season hit a massive revelation in its midpoint, that allowed for the show to be changed almost entirely afterwards.

After Red John was killed, the Director of the CBI organization was revealed to be a traitor working with him, seeing the entire thing shut down. As such, halfway through the sixth season, The Mentalist saw a two-year timeskip. This timeline change was followed by a change of setting, cast - despite keeping the same main characters - and a shift to a standard crime-of-the-week format.

9 The Bill Tried To Reinvent Itself

The uniformed police officers of the Bill TV show

The Bill was one of the UK's longest-running shows, a serialized police procedural with thousands of episodes. The Bill followed an ever-shifting team of policemen in crime-of-the-week scenarios. It has undergone several retools in its long life, most notably towards full serialization - with each episode following two crimes, one of which would end on a cliffhanger each episode.

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Most of these, as with most retools, occurred between seasons. In its twenty-fifth season, however, beginning with mid-point episode Live by the SwordThe Bill took a drastic turn. The Bill set itself after the 9pm watershed to show more adult content, replaced the theme tune, and axed several characters. The changes proved controversial and didn't bolster the show as hoped.

8 Angel Has A Short Retool To Close Itself Out

The Angel Investigations team confront Angel about his seeming corruption in 'Power Play' Angel show

Buffy spin-off Angel retooled itself more than once, changing from a supernatural noir-themed show to an arc-based urban fantasy series in its second season. In Angel's fifth season, it changed to a monster-of-the-week format, which also saw the cast working for their old enemies, Wolfram and Hart.

When Angel was suddenly cancelled in its fifth season, however, Angel made several changes that had all the hallmarks of a retool. The ongoing storyline about the slow corruption of the team at the hands of Wolfram and Hart was dropped. Lindsay became a major character on the team's side. Finally, a sudden plotline about an ancient conspiracy was introduced to provide a satisfying final foe.

7 Ellen Saw Cast And Setting Shifts

Ellen DeGeneres and Mary Tyler Moore in the bookshop set Ellen sitcom

The sitcom Ellen is best-known for making Ellen DeGeneres a household name, and for her seminal coming-out moment in The Puppy Episode. Initially, however, it was conceived as a gender-flipped version of Seinfeld known as These Friends of Mine, with DeGeneres being the show's Jerry Seinfeld-esque focal point.

Midway through Ellen's first season, it went on hiatus to adjust criticisms. Although the name would stay for the remainder of the first season, some characters were dropped, and others were introduced. A chunk of the action moved to the bookshop where Ellen worked, increasing her prominence.

6 Fear The Walking Dead Somewhat Undoes Its Pre-Season 4 Retool

Morgan Jones uniting with the other cast members Fear the Walking Dead

Owing to its often-tumultuous critical response and viewership, Fear The Walking Dead has been through more than one retool to try to address criticisms. Fear The Walking Dead was retooled to stand out in its own right from its parent series, and to freshen a stale premise.

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The beginning of season 4 saw one of Fear The Walking Dead's most drastic retools, changing the show's focus to The Walking Dead character, Morgan Jones, travelling across America. It also added multiple plotlines and a Western tone. Halfway through, the jarring nature of these changes is eased by having Morgan come across some of Fear The Walking Dead's older characters. At which point, its theme and format changes again to better-resemble older seasons.

5 Emmerdale Took The Lethal Approach To Reinvention

Characters watching the fateful plane crash in Emmerdale

Sometimes, writers decide to shake up a show with a mass casualty event. This gives instant storylines about mourning and broken families, allows for the introduction of new characters, and more subtle changes to fly under the radar.

British soap opera Emmerdale, initially known as a more light-hearted sitcom about farmers, completed its transition to a darker, edgier show with the Beckindale Air Disaster. Featuring a plane crashing into the village of Emmerdale's setting, four major characters were killed and half of the village demolished. Accompanied with tonal and story changes, Emmerdale became nearly unrecognizable.

4 It's About Time Flips Its Premise On Its Head

The astronauts meeting the cavemen in It's About Time

In many cases, a retool derives from a change in the show's very premise, what many would consider a drastic step. Comedy show It's About Time followed a pair of astronauts who were standed in prehistoric times. Focusing on them being out of place and attempting to civilize the cavemen, it seemed a solid plot.

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Midway through the season, the astronauts' shuttle was fixed, and they returned home. They brought with them a pair of caveman stowaways, and It's About Time became about the cavemen being out of place, and attempting to hide them from the government. It's About Time would only see one season, leaving almost half of its entire existence post-retool.

3 The Studio Mandated A Change To Sleepy Hollow's Format

A promotional image for Sleepy Hollow Season 2

The first season and a half of Sleepy Hollow follows an expanded version of the story the titular town is best-known for: the efforts to stop the Headless Horseman and his master from bringing about the end of days.

Following a time of this one running plotline, Sleepy Hollow sees a shift in its second season towards a monster-of-the-week format, and instead following the main characters as they dealt with a bevy of supernatural threats. This retool had fewer ongoing storylines. Executives hinted that they mandated the change, which for a time buoyed Sleepy Hollow's critical reception.

2 The Event Dialed Down Its Similarities To Lost

The main characters of the Event TV Show

The impact Lost had on contemporary television cannot be understated, with many shows attempting to emulate aspects of the Lost's runaway success. The Event follows a group of people intertwined in a series of happenings that were surely going to lead to the unexplained titular event. The Event attempted to emulate Lost's confusing storytelling, focusing massively on flashbacks and nonlinear story lines.

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This resulted in low ratings for The Event, with many finding it lacked the substance to support the style, amongst smaller issues. Following a hiatus, The Event resumed its first season in a much more linear style. This, coupled with a general improvement in the writing, carried it to a second season.

1 Agents Of SHIELD Had No Choice But To Retool

Coulson and May confront HYDRA in Agents of Shield 'Turn, Turn, Turn'

One of the perils of creating a show in a shared cinematic universe is that the events of that universe may force your hand with storytelling. Agents Of SHIELD is notorious for its initial difficulties as part of the Marvel Cinematic UniverseAgents Of Shield debuted shortly before Captain America: The Winter Soldier, which destroys SHIELD in its climax.

As a result, after sixteen episodes of the team working within the organization, the episode Turn, Turn, Turn shows the destruction of SHIELD. The characters were forced underground, without the backing of the UN security council. Despite a shaky start post-reboot, Agents Of Shield would eventually come into its own.

NEXT: 5 Reboots that Were Better Than The Original (& 5 Old Shows That Should Be Rebooted)