A schoolboy meets a schoolgirl on the railway station and gives her umbrella. You're right if you immidiately thought about love story, because that's how romances begin in recent anime. The "The Tunnel to Summer" is full of such cliches and you shold be ready for them while watching this anime like you are ready to the fact that there are little to no differences between fast foods' menus. The boy is your stereotypical male lead from modern anime: average in all aspects possible, anxious, whiny, hyper-reflective, impulsive, obsessive, self-absorbed, lacking common sense, intellectually and emotionally dull. In other words he is typical psychasthenic. The girl also matches current tropes - she is determined, rational and brave. Overall that's your another modern cliche with gender roles inversion. How refreshing indeed. The boy and the girl are in some way alike because both of them have problems with their parents and struggling to find a way out of their current situation. To cope the boy clinged to the past and the girl clinges to her dream of becoming mangaka.
That's where fantastic element comes into action. The boy finds a mysterious tunnel where you can find something that you lost in the past. The tunnel has a different time flow with hours there being years in the real world. So the boy and the girl begin to explore the tunnel in order to gain their wishes from it. The boy wants to ressurect his deceased sister while the girl wants to find in the tunnel a talent for manga that she thinks she is lacking (no logic here, since you can't find there something you didn't have - yet another clumsy sacrifice for the plot progression). In the process they fell in love with each other but since it's an anime they are unwilling to admit and reveal their feelings untill it's almost too late. The moral here is that you should not cling to the past but look into future. End of story. There are many plotholes and dumb acts in the process but overall that's it because this boring and tedious anime indeed ends abruptly.
The name "The Tunnel to Summer" is most probably a refference to Heinlein's "The Door into Summer" where the protagonist is trying to find a way out of the disastrous situation he stucked in. Heinlein's novel is also dealing with time shifts and a love which survives through years. But where in "The Door into Summer" the main character is actively searching for his "door into summer", putting all his efforts and ingenuity into this, the protagonist of "The Tunnel to Summer" is passively wasting his life and endagering his love by obsessively clinging to his past. Giving the tendency of anime protagonists to became more and more dumb and mentally disordered it's a good thing he realizes his problem after all. But really, he is not the best example to follow for the young adults. Heinlein's protagonist is much better suited for this role. Not to mention that "The Door into Summer" is a much more enthralling and imaginative read.
But "The Tunnel to Summer" has its pros. First of all it has great visual both in terms of drawing style and the animation. The main characters are aesthetically appealing and likable despite their density. Also this anime has positive moral message and happy ending, which is a great feat in the times of hypersentimental and decadent storytelling.
