Think Timecop meets Looper.

So in the near future, someone invents time travel and manages to relatively keep it a secret. Amongst the uses, the government finds for it is a form of witness protection.

They take the person either into the future or the past at a random time and place, and there is someone there to pick them up and bring them to whatever time and place the trial happens.

So much to play with here.   What happens when the system breaks down and no one is there to pick up the witness.  What happens if the whole system is compromised by the mob or another criminal organization...