Graduation Day was not an official Infinite Crisis tie-in, rather it’s subject matter and storyline was – like Identity Crisis – co-opted by IC and woven into the story.
Unknown to her or her colleagues in the Titans, a robot from the future – who will become known as Indigo – appears in the present, damaged and looking for a suitable hiding place.
The Titans, meanwhile, are approached by a company wishing to bankroll them; at the same time, the same company are offering Young Justice the same deal and, inevitably, the two teams meet.
Indigo, attempting to find a compatible host, attempts to merge with the Metal Men but finds them unsuitable. She then teleports into the Titans’ meeting and attempts to attach herself to Cyborg.
Attacked by both teams who take her sudden appearance as an attack, Indigo initiates defensive measures, wounding several of the heroes. Damaged herself, she eventually teleports out, leaving the teams to lick their wounds while Cyborg, who had been connected to her, realises that she had not meant to hurt anyone.
Issue #2: The teams take Cyborg and the other injured heroes to hospital while Indigo searches for a compatible host. A military outpost is targeted by her and, despite the soldiers defending it, she manages to find an inert Superman robot.
Both the Titans and Young Justice speak within their teams concerning their fears, not only of how easily Indigo had defeated them, but their own insecurities and worries about their team mates.
As Cyborg, now recovering in hospital, again tells the teas that Indigo’s attack was unintentional, they hear a news report of her attack on the military compound. They arrive and find what appears to be Superman doing something to the body of Indigo. As the Titan member Omen approaches, Superman turns and breaks her neck, killing her.
Turning, the teams see that it is actually a Superman robot.
Issue #3: Both the Titans and Young Justice are attacked by the Superman robot which has almost the same level of powers as the hero they are modelled after.
As the teams struggle, Indigo, severly damaged, offers to help the heroes if they will repair her and, despite Robin’s misgivings, Arsenal gets to work. Whiel the battle rages around them, they are joined by someone who later turns out to be Shift, a clone of Metamorpho; at this point he believes himself to be Metamorpho.
Donna battles the Superman robot to a standstill but before she can destroy it, she is killed by its heat vision. Too late to save her, Indigo manages to shut down the Superman robot before collapsing herself.
Later, at Donna’s funeral, Young Justice and the Titans effectively disband, though the teams would reform in a new Teen Titans and Outsiders.
Donna, meanwhile, appears to wake up on an alien world, armed with sword and shield. Hearing the sound of battle she runs toward it.
Ramifications from this storyline were featured in the Return of Donna Troy and the Teen Titans/Outsiders crossover The Insiders.