The new semester starts and on the bus to school Saku notices a tall man also getting off at the school. During the welcome back ceremony, the man that Saku saw earlier is introduced as a new teacher named Nozomu Soma who is in charge of Saku's class and world history. Mihoshi calls him "So-nii" in front of everyone and rumors abound as to what their relationship is. Later, Soma becomes the astronomy club's adviser and he explains to the others how Mihoshi's father was his professor at college and also the club adviser for their astronomy club. Soma wants the club to do a planetarium for the upcoming cultural festival and they get in the top three in a popularity contest in exhibits, the club will get more funding. Saku becomes annoyed at Mihoshi's and Soma's relationship and is approached by Fumie to help with the literature club.

Despite initially going to refuse, Saku ends up agreeing to help the literature club. Mihoshi acts cheerful without him there, but is still troubled by his absence. Meanwhile, the astronomy club has their hands full with making the planetarium. Mihoshi suddenly hugs Saku one day after school and rumors spread after some athletic club members see them together with Soma. Fumie becomes concerned that Saku has been helping the literature club too much and that he may be concerned about the rumors regarding him, Mihoshi and Soma. Hime gives Saku a journal she compiled about the planetarium's construction. Saku feels re-energized about the astronomy club and goes back to help.

The cultural festival begins, though initially no one is coming to see the planetarium. After Haruko from the city planetarium arrives, she helps to bring in the people and the planetarium is well received by viewers for the rest of the day. Mihoshi thanks Saku for everything he has done for the club and is thankful that she has not caused him any trouble while he was helping the literature club. After the festival, the club members are distraught at the thought of dismantling the planetarium, but eventually take it down. The astronomy club was not able to win any prizes for the festival and thus still do not have enough money to even buy a telescope. Sayo takes the club to her family's Buddhist temple and perform a small moon festival. The astronomy club receives a video letter from the president of the astronomy club of Nogishiro High School in the same prefecture as Sōei High School inviting the club to the Autumn Stargazing Conference of the Prefectural High School Astronomy Network.

Mihoshi and the other members are excited about the Autumn Stargazing Conference, and are surprised at the size of Nogishiro High School along with the number of members in their astronomy club. After the sun sets, everyone splits into groups and the process is explained by Nogishiro's astronomy club president Ayumi Ōmi; each group is free to stargaze until the moon sets, upon which each group will begin counting visible stars in a patch of sky to measure the overall brightness of the sky. Ayumi asks Saku to help on patrol around the school and they come across Hime and a second-year named Musa after she overreacts and thought he was trying to assault her. Afterwards, everyone goes back to the roof now that the moon has has set, and Ayumi confesses to Saku that her astronomy club actually only has nine members in it and mostly everyone else is from other clubs. Takeyasu Roma does a narration of the autumn sky myth which everyone loves. Ayumi later confronts Roma and reminds him that they were once members of the same astronomy club at Goshima Science Museum.

After overhearing Mihoshi and Soma about plans together on the weekend, Saku, Hime and Edogawa follow them around town to find out what is going on, and initially they are out on a date. They eventually follow them to the grave site of Mihoshi's father who died in a car accident and everyone pays their respects. Saku, Hime and Edogawa feel awkward around Mihoshi after this. Fumie tries to give Saku a bookmark souvenir from her school trip to Hokkaido, but her glasses are broken after Saku protects her from a stray baseball. Saku offers to walks her home and on the way they run into two members of the literature club who to take her the rest of the way. Saku gets Fumie's help to set up a shooting star viewing event at school for Mihoshi, who is overcome with emotion over it.

Saku's astronomy club participates in a winter stargazing camp with the girl from Takami Girl's School, who also participated in the Autumn Stargazing Conference. Saku, Edogawa and especially Hime do not dress warm enough, and while Saku and Edogawa brought more warmer clothes, Hime cannot dress in layers with the extra clothes she brought. After spending the first day freezing, Hime goes out at night to go to a local convenience store, but it is already closed by the time she gets there and a blizzard is blowing in. Hime tries to get back to the cabin, but quickly gets lost. The others notice she is gone, and also suspect Mihoshi went out to search for her. With Hime close to freezing to death, Mihoshi finds her and the two wait out the blizzard in a small snow cave. They find they were not far from the cabin and while they are scolded for what they did, everyone is relieved to have them back safely.

The astronomy club members wake up late the next morning and first spend some time warming up in the outdoor baths. Ayumi Omi notices Roma stressing over studying for university entrance exams, and takes him to buy a study book and then have dinner at a fancy restaurant. The astronomy club members wait until the moon sets on the third night and spend their final night in the mountains stargazing and taking photos. The members use some eyepieces Mihoshi brought to find Saturn, which is an entrancing sight for everyone. The following spring, Mihoshi has succeeded as club president after Roma graduated and entered into university with Ayumi. Everyone puts in more effort this year for recruiting members, since they are now one member short, and are again in danger of getting disbanded.