During the events of the Mass Effect trilogy, Commander Shepard recruits Tali'Zorah the Quarian to his/her team, but in 2185, Tali received a shocking message: she was being accused of treason against the Quarian Migrant Fleet.
Tali could think of no good reason to be on trial, and the cryptic message was saving the full details for the trial itself (it probably wasn't safe to transmit them across space). So, Commander Shepard ferries Tali back home to the fleet so she can face justice. No one aboard the Normandy was prepared for what came next.
The Geth Threat
When Tali arrives at the Rayya (a ship of the fleet), she is greeted by a security captain who confines her to the Rayya for the time being. Fortunately, Tali is allowed to have Commander Shepard legally represent her at the trial, since Quarian laws (which are streamlined and pragmatic) recognize Shepard as Tali's captain. Tali also meets her aunt Raan before the trial, but Raan, an admiral, must be excused from the trial, and she urges Tali to enter the chambers and begin.
Three admirals are ready to judge Tali's case, and the charges are made clear at last: endangering the fleet with live Geth. Tali and Shepard alike are bewildered; how could they have sent active Geth to the fleet? Shepard promptly argues that Tali is innocent and pleads not guilty for her. More details are laid out, and evidently, the many Geth parts and pieces that Tali had sent to the fleet had somehow reassembled themselves and formed fully-functional Geth. Tali had been scrounging for Geth parts for her father Rael'Zorah's research, but things got out of hand. Worse yet, the active Geth took Rael'Zorah's lab ship (the Alarei) by force, and most likely killed everyone aboard. Tali is allowed to form a squad to dock with the Alarei to retake it and find out what happened to Rael'Zorah.
Shepard, Tali and a third squadmate retake the Alarei as intended, and they are searching for evidence of how all this could have happened. Along the way, Tali happens upon a tragic sight: her father's body. One last recording involves a short explanation and apology for all this, and a little deeper in the Alarei, Tali finds the truth at last. And it has grave political ramifications for the entire Quarian race.
The Quarian Proxy Battle
The truth is made clear: Rael'Zorah knowingly reassembled the Geth pieces aboard the Alarei so he could test weapons against them. The Geth formed stronger networks than the Quarians could handle, and thus, the Geth broke free. This video evidence will clear Tali's name, but in exchange, it'll ruin Rael'Zorah's good name and deepen the political divides among the Quarian people. In Rayya's courtroom, Shepard and Tali prepare to finish their legal case, but there is an important decision to be made: do they reveal their evidence? It might be Tali's only shot at clearing her name.
From here, Shepard may choose to present that evidence, and as expected, Tali will be declared innocent, Rael's name will be blackened for all time, and the Migrant Fleet's political divisions will be deepened. Or, if Shepard can find the right words, he or she will argue earnestly to the Admirals that Tali's past makes her innocence and good intentions obvious. What is more, Shepard has realized that this debate about the Geth and Tali is a proxy battle for the main debate: whether or not the Migrant Fleet should arm itself and retake Rannoch (the Quarian homeworld) from the Geth. The admirals are testing the waters with this small-scale proxy battle, and Shepard can call them out on it and thus strengthen Tali's case. Most likely, this will work, and Tali gets it both ways: she is declared innocent, and her father's legacy is untarnished.
Taking that last route will have positive ramifications later on, such as the possibility of uniting all the Quarian people and bringing them to peace with the Geth, similar to how Shepard can cure the genophage and bring peace to the Krogan, the Salarian Union and the Turian Hierarchy. Meanwhile, Legion, the ambassador of the Geth, can make the most of this by giving its life to strengthen all the Geth and use them to help rebuild Rannoch for the Quarians. By contrast, revealing that evidence to clear Tali's name will result in a fractured Quarian fleet that may be unfit to live peacefully with the Geth in the future.