After this third defeat, he had a dream in which a ship came to him. In the morning, he came across a ship; he boarded it and set sail for weeks. Inside the ship was the body of a dead woman clutching a letter. One day, Galahad came to them and lived in the ship for months. They had adventures with animals and learned to care for one another.
Galahad is a strangely pious knight, whose goodness means he is almost unreal. However, while they are together for six months, Lancelot—for the first time—gets to know his son.
One Monday, they arrived at a forestland where a white knight was waiting to take Galahad to the Holy Grail. Lancelot knew he would not be taken too, but asked Galahad to pray for him. The ship took Lancelot out to sea again alone. He prayed to learn some tidings of the Holy Grail.
Lancelot's unspoken emphasis about the reason he cannot be taken to the Holy Grail is his relationship with Guenever that has forever besmirched his soul.
Finally, the ship came alongside a castle when it was black as death. Lancelot put on his armor and snuck into the castle until he came to a chapel. It was the most beautiful chapel he had ever seen. He couldn't go inside, but could see Galahad, Bors and Percival, and knights from other lands; and on a silver table was the Holy Grail.
Although Lancelot was not granted to find the Grail, he was allowed to see it and know that it has been found. This is arguably the closest a knight can come to the Grail, and still be an imperfect soul who can live on (as opposed to Galahad and Percivale).