The thing in the castle

The old professor Bunzberger sits on a park bench and lets his imagination roam. He is kidnapped by two pensioners in a veteran's home and is prompted to put the finishing touches on a time machine designed by the elderly. First he refuses to take part in the experiment for ethical reasons. He discusses with a kidnapped colleague and friend about the meaning of life and the consequences that stopping aging can have. Finally he gives in and can be rejuvenated with the machine. But he is not satisfied with the result because the rejuvenation has only taken place externally. Awakening on his park bench, he returns to reality.

https://www.defa-stiftung.de/filme/filmsuche/das-ding-im-schloss/