Counterweight

S2 E14: A group of carefully selected passengers board a space ship for a simulated flight to a space ship for a simulated flight to a distant planet. A panic button is the only contact they'll have with the outside world for the duration. Pressing it will end the flight. The long confinement plays upon their nerves and the interplay of their emotions. Gradually fear spreads throughout the group. Fear gives way to panic when an alien influence actually invades the ship. The alien power fears that its own security is in jeopardy if the research continues. Seizing one of the passengers the power tries to force him to push the panic button to end the experiment.