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On a recent trip to the roof to inspect my roofjack as part of a furnace diagnostic, I noticed that a shingle was missing from the marriage line. To my surprise, I found that there was no paper/plastic/felt underneath the missing shingle -- just a 1" x 6" opening that went all the way down to the floor. To make matters worse, the plywood underneath appears weather-worn. I have no idea how long that shingle has been missing, and although I live in fairly dry northern California, we have had an unusually rainy autumn this year.

What I need to know is whether I need to put any kind of liner underneath the new shingle. It looks like there may have been something there before, but it's hard to tell. Also, is there anything else I should do or know before nailing a new shingle on?

Thanks in advance!

You are actually missing two cap shingles. It looks like they must have been blown off at some time. If it were my place, I would take off all the cap shingles over the marriage line and replace, as those appear to be stapled on, rather than nailed. I would think there should have been a layer of felt lapped over the joint, before the setup crew installed the ridge cap. At this point, again if it were me, I would remove the cap shingles, install a layer of felt along the whole ridge, slightly narrower than the width of the cap, and replace the whole length, with nails rather than staples. Since you are in a relatively dry climate, that may hold you until time comes for a whole re-roof.
We'll see what JD, the expert, has to say.

Yes, what he said. And that is my "expert" opinion.

Actually, when the two halves were put back together they should have had to install the last three courses on each side. This allows them to overlap the felt for two full layers. Two layers of felt (15# or 30#) is a minimum requirement. Ice & Water Shield only needs one layer.

At this point, I would add the cap shingles as Commodore said. If your shingles are in need of replacement, get the whole roof done properly.