Yes, INTERVENES is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, T 1, E 1, R 1, V 4, E 1, N 1, E 1, S 1.

Is it a valid word?

INTERVENES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

get involved, so as to alter or hinder an action, or through force or threat of force
be placed or located between other things or extend between spaces and events
occur between other event or between certain points of time

Also: step in.