How real is a shadow, when it is so filled with emptiness? This question, and the theme of innocence, forms the running thread of Athol Fugard’s most recent play, “The Shadow of the Hummingbird.” As both writer and actor, the production at the Long Wharf Theatre signifies Fugard’s return to the stage following a 15-year absence.

“The Shadow” centers on two characters: Oupa (Fugard) who is old and weighted with knowledge, and his grandson Boba (Aidan and Dermot McMillan) who is young and vibrant. Oupa and Boba are noticeably different: While Boba rushes to catch a glimpse of the bright hummingbird feeding outside the window, Oupa is satisfied looking only at its shadow, flitting across the wall. When Boba comes to visit Oupa for an afternoon, the two embark on a journey to examine the importance of knowledge and the cost of gaining it. It is a story about generations: How age separates two people and how their love can bring them back together.

The play opens in Oupa’s living room, set in present day Southern California. The home has a cozy feel to it: bookshelves crammed with notebooks and journals, a worn couch near the center of the stage, a basket of fishing equipment in the corner. Oupa enters the stage looking for his glasses. “Spectacles, spectacular!” He announces. He is charming and eccentric, and as he finds his glasses he begins rifling around for a journal entry. His constant mutterings of the word “shadow” are the only clue we are given to what the entry might be.

As Oupa searches through the piles of notebooks, he reads some of them aloud. These entries, which are taken from Fugard’s personal journals, range in their scope. Some are lighthearted, like descriptions of birds from his home in South Africa. Others are haunting; one entry recalls Oupa desperately opening windows to dispel the darkness within him.

While Oupa reads, we are moved by his frailty: He drops his books, he has trouble getting up. At one point he barely makes it to the couch before he collapses in a heap. The set aptly exposes Oupa’s worn state: As he moves from the cluttered desk to the fraying couch, he seems to be aging and weakening. As props, the journals reflect his life; they are eclectic and worn, they are frayed on the edges and peeling at the seams. In the same way, the shadow entry reflects Oupa’s vulnerable condition, in which he questions what can truly be known about reality and reflects on the shadow of a tree limb on his bedroom wall. Oupa knows the shadow is nothing, yet from its movements he cannot help but feel that it is real.

Boba breaks through the play’s darker mood, rushing onto the stage and yelling, “Oupa! Oupa!”. His grandfather responds by commanding him to “draw his sword,” and the two engage in an imaginary play-battle. But after the mirth subsides, we see hints of a tense relationship between Oupa and his son-in-law (Boba’s father), whom he unapologetically deems an idiot. But for Oupa, it is clear that Boba is the center of the world. The actors’ tender portrayal of the characters captures with sincerity the essence of a grandfather-grandson relationship. When Oupa rubs his arthritic hands, Boba immediately moves to massage them. When Boba’s eyes wander over the bookshelves, Oupa tells him the cookies are under the Bible without missing a beat. There is a synchronicity between the two actors that gives their dynamic an added depth.

The relationship between the two also mirrors that of teacher and student. Oupa tries to teach Boba about knowledge, quoting Tolstoy and introducing the child to Plato. When Boba fails to understand the allegory of the cave, Oupa grows upset. Boba’s indifference to his teachings makes Oupa frustrated, and he tries harder still to drill into Boba the difference between reality and knowledge.

At the end, the play returns to shadows. Oupa tells Boba that his only dream is to be able to look at shadows like he once did as a baby. He gazes at the shadow of the hummingbird because he wants to regain the innocence that would allow him to think catching a shadow is possible.

The play concludes on a dim note, but there’s a bright kernel to be found. Despite Oupa’s learned nature, what he really wants is to look at the world like the young and naive Boba. The knowledge of the shadow’s transience doesn’t stop him from admiring its dance.