So this au plays two weeks after the kraang invasion where Donnie was the only survivor

In the invasion when Leo got trapped in the prison dimension when Mikey tried to open the portal the power instead shattered his body

and Leo was stuck in the prison dimension and died bleeding out when Prime impaled him

So after the event, it was only Raph and Donnie who survived

And even though it was hard for them they still had echouther

Until 5 days later Raph's eye has started to get worse

Donnie was desperately trying to save him, he was the only thigh he could still hold on to but it had no use

Two days later Raph passed away leaving Donnie truly alone

So Donnie, not knowing what to do started to try and save his only brother who he hadn't seen dead

Leo

Not knowing he is already dead, his mission has become to bring him back