d hero plasma

destiny hero plasma's effect lets him equip an opponents monster once per turn one card at a time. does that mean that at the end of the turn the monster returns to the opponent.

No.

A monster equipped to Plasma will stay equipped until Plasma is no longer face-up or the equipped monster is destroyed by a card like Mystical Space Typhoon or Heavy Storm.

You don't assume that there are conditions or effects that are in play unless explicitly stated. In this case, Plasma doesn't state that the equip card changes control back to the owner of the card, so it's assumed that it remains equip to Plasma until detached by card mechanics or destroyed by an effect.

Confirming the above statements. The monster(s) equipped to Destiny Hero Plasma will stay equipped as long as Plasma is face-up on the field or the monsters are destroyed by cards such as Mystical Space Typhoon or Heavy Storm.