DS9 "Time's Orphan":
DAX: It's some sort of time portal. From the chroniton signature, we think it sent Molly about three hundred years into the past.
KIRA: I wonder what this place was like three centuries ago.
DAX: Odo contacted the Bajoran Archaeological Institute to see what he could find out.
KIRA: How do we get Molly back?
DAX: Well, **first we have to find a way to reactivate the portal. It's been abandoned a long time and it was destabilised when Molly fell through. The problem is, we're not sure how this technology works. ** KIRA: Well, something tells me the Chief isn't going to stop until he figures it out.
(Machinery is thrumming.)
O'BRIEN: Transporter interface link.
O'BRIEN: DNA acquisition sensors.
BASHIR: Locked in.
DAX: We're as ready as we're ever going to be, Chief.
O'BRIEN: Okay, let's do it.
(The purple swirly appears in the now-upright portal.)
DAX: The field's destabilising.
O'BRIEN: We're losing power.
DAX: Temporal field established.
BASHIR: We've got a DNA lock. It's her.
(They beam in a ragged young woman wearing Molly's bracelet.)
Why don't they just do what happened at the end of the episode and walk through the time portal grab Molly and walk her back through?