Marvel Team-Up 61 September 1977
This story doesn't feature the Fantastic Four, but the Human Torch was a regular co-feature. Indeed Marvel Team-Up started as a series featuring Spider-Man and the Human Torch before it became a Spider-Man vehicle. A series with guest-stars was considered the best way to avoid continuity problems for characters who already appeared in their own series. After three issues together, Spider-Man started teaming-up with other characters. In 1974, when Marvel launched Giant-Size Spider-Man, once again he was teamed up with other characters so as to write continuity-free adventures. Therefore, for its five-issue duration, on the months when the quarterly GS Spider-Man was released, the Human Torch took Spidey's spot as guest in Marvel Team-Up (#23, 26, 26, 32, 35).
The Equinox mentioned by Spider-Man is a reference to the previous Marvel Team-Up issues #59-60. This story takes place just after that one.
The villain is the Super-Skrull, a shape-shifting alien with all the powers of the FF. This is therefore an opportunity for Byrne to draw their powers in action. Reed Richards and the Thing even appear in flashback. This is likely the reason that warranted the inclusion of this issue.
NYPD Captain Jean DeWolff was created earlier in this same series by Bill Mantlo and Sal Buscema. She was the first woman police captain of the Marvel universe. She mainly appeared in stories by Mantlo and Claremont (in MTU) in the 70s.
Lt Scarfe was created by Chris Claremont for the Iron Fist story in Marvel Premiere #23.
DeWolff and Scarfe both appeared in MTU #60. Here, Scarfe mentions the Equinox case is wrapped up as he is subject to psychiatric evaluation at Bellevue hospital. This is a progressive notion in that this villain.
Lt Kris Keating was created in Defenders #44, February 1977. He also appeared in Defenders #51 published the same month as this issue of MTU. Was that why Keating is unavailable to deal with the crisis? Was Claremont aware of what Kraft was writing? Whatever the case, Claremont did his homework on the NYPD of the Marvel universe.
Both DeWolff and Keating would be revealed by Peter David in the late 80s as having been murdered, while Scarfe recently (in the crossover Shadowland) became a criminal, even threatening his former partner, Misty Knight. There is no respect for secondary supporting characters.
The flashback with Tigra wraps up a storyline from the cancelled Marvel Chillers. Tigra started featuring in #3 under Tony Isabella. Claremont wrote #4, pitting her vs Kraven (as he would again in MTU #67). Isabella returned for #5 and #6, introducing Joshua Plague, revealed as Super-Skrull in the latter illustrated by Byrne. Issue #7 was by Shooter where Super-Skrull is trapped by the soul-catcher (which appears on page 1 of MTU 61). Claremont and Byrne both returned to Tigra in MTU #67.
Actually, under Claremont Marvel Team-Up featured a record number of co-feature female characters (#59-60, #62, #64, #67, #77, #79-82, #85, #88) as he demonstrated early on his knack for strong female characters.
Carol Danvers first appeared as a Security Officer in Marvel-Super Heroes #13 (March 1968) as a supporting character for the Captain Marvel feature. Becoming a female version of that character in Ms. Marvel #1 (Jan 1977), she started a late 70s trend that would include Spider-Woman and She-Hulk. Claremont started writing her adventures in Ms. Marvel #3 (March 1977). Those three panels announce her appearance in next issue.
Here, the Super-Skrull receives his powers from an energy beam as in FF #18. While this is mostly forgotten by other writers, the idea may have been suggested by Byrne who mined the early work more than anyone else.