Michelle Scarabelli takes off the AlieNation "Newcomer" spots and appears as "Jenna", an eccentric crew-woman who becomes attached to Data like a tether.
It's very interesting to watch Data's process: of course he has to get everybody else's opinion before he acts. He gets great advice from Guinan and Riker.
When watching Data interact with the O'Briens in 10 forward, we see that even though he cannot participate in humor internally, he is finally able to understand how to make a joke and make it funny, even though he himself doesn't understand the joke or can actually laugh at it. That's what makes him the most humorous member of the crew, because what for for him is an innocent question can come off with hilarious results because of the way he asks it.
But this situation is a little bit more complicated than merely figuring out what humor is, this is in fact Data's first girlfriend, not counting his interaction with Tasha in season one.
In the meantime the ship is exploring a very interesting looking dark matter nebula. This provides the backdrop for Data's exploration of sex and relationships.
The two subjects counterbalance themselves in this episode. While Data's meanderings give us the humor, there is actual danger for the Enterprise that is not initially seen. Some bad things have to happen before they actually understand the level of danger they are in.
The two issues are resolved, not too well for Mr. Data. In theory, my theory: Data should not have created a subprogram to govern his relationship, he actually was doing fine without it. This is just another example of how Data's interpersonal experimentations fail with hilarious results, hilarious for us, Data of course has no feelings.
This is what he always says, but after Jenna leaves him, and he deletes the program, we are left with an image of Data picking up his cat Spot and holding it. That image says a lot. That although Data does not have feelings in the way humans have feelings, he does have some Positronic version of affection. He loves his cat, we know he loves his cat. I suppose Data has become "used to Spot's sensory input information".
Patrick Stewart "captained" this particular episode as well as Picard captains the Enterprise.